It would appear that, in future, new battery systems will increasingly be used both in stationary applications (for example in wind power installations) and in vehicles (for example in hybrid and electric vehicles), with very stringent requirements being placed on said battery systems in respect of reliability.
The background to said stringent requirements is that failure of the battery can result in failure of the entire system. By way of example, in the case of an electric vehicle, failure of the traction battery leads to a so-called “breakdown”. Furthermore, the failure of the battery can result in a safety-related problem. In wind power installations, for example, batteries are used in order to protect the installation from impermissible operating states in a high wind by virtue of rotor blade adjustment.
The block diagram for a battery system in accordance with the current prior art is illustrated in FIG. 1. A battery system, denoted as a whole by 100, comprises a multiplicity of battery cells 10 and a charging and isolating device 12, which comprises an isolator switch 14, a charging switch 16 and a charging resistor 18. In addition, the battery system 100 comprises an isolating device 20 having an isolator switch 22.
For safe operation of the battery system 100, it is absolutely necessary for each battery cell 10 to be operated within a permitted operating range (voltage range, temperature range, current limits). If a battery cell 10 is outside these limits, it must be removed from the cell network. When the battery cells 10 are connected in series (as shown in FIG. 1), failure of a single battery cell 10 therefore results in failure of the whole battery system 100.
In particular in hybrid and electric vehicles, batteries using lithium-ion or nickel-metal hydride technology which have a large number of electrochemical battery cells connected in series are used. A battery management unit is used for monitoring the battery and is intended to ensure not only safety monitoring but also the longest possible life. For this purpose, the voltage of each individual battery cell is measured together with the battery current and the battery temperature and a state estimation (for example of the state of charge or of the state of aging of the battery) is made. The evaluated data are passed on to actuators which can act on the cells in a corrective manner (for example by charge balancing or temperature correction). At the same time, the battery management unit provides the user with up-to-date information in respect of the state of the battery. In order to maximize the life, it is helpful always to know the present maximum capacity of the battery, that is to say the maximum electrical power which can be output or drawn. If this capacity is exceeded, the aging of the battery can be markedly accelerated.